Recently, to reduce atmospheric contamination by exhaust gases from automobiles, automobiles are manufactured based on a study for securing driving power by using an internal combustion engine and/or an electric motor. Therefore, the automobiles have evolved in a sequence of hybrid automobiles, plug-in hybrid automobiles, and electric automobiles.
The hybrid automobiles and the plug-in hybrid automobiles include an internal combustion engine, an electric motor, and a battery pack. The electric automobiles include an electric motor and a battery pack without an internal combustion engine.
The battery pack includes at least one battery module and includes an air-cooled or water-cooled cooling mechanism.
For example, JP2013-038001A discloses a battery pack including a plurality of cooling fins, a plurality of battery cells, and a heat-absorbing body. The cooling pins have a plate shape and are erected on the heat-absorbing body and arranged in a line. The battery cells are located between the cooling fins on the heat-absorbing body. The heat-absorbing body receives a coolant through one side and discharges the coolant through another side.
The cooling fin contacts the battery cell and the heat-absorbing body. The battery cell generates heat during repeated charging and discharging operations. Heat from the battery cell is conducted to the heat-absorbing body through the cooling fin. The heat-absorbing body exchanges heat with the cooling fin through the coolant.
Meanwhile, in the array of the battery cells, the temperature of the battery cell located at the central portion rises relatively faster than the temperature of the battery cell located at the outer portion. This is because heat generated from the battery cell located at the outer portion is transferred to the battery cell of the central portion and accumulated there. The accumulation of heat increases a deterioration speed of the battery cell located at the central portion. Consequently, the life of the battery pack becomes shorter than design specification, which shortens an exchange period of the battery pack and thus becomes a cause of an economic burden.
Meanwhile, U.S. 2012/0009455A discloses a battery module including: a plurality of adjacent battery cells; heat transfer sheets located between the at least several battery cells and configured to exchange heat with the battery cells; a heat dispersion member coupled to the heat transfer sheets and configured to exchange heat with the heat transfer sheets; and a heat dissipation member coupled to the heat dispersion member and configured to exchange heat with the heat dispersion member.
In the battery module disclosed in U.S. 2012/0009455A, an auxiliary heat transfer sheet is additionally inserted between the heat transfer sheets at the central portion and thus the thickness of the entire heat transfer sheet at the central portion is relatively thicker than the thickness of the heat transfer sheets at the outer portion. This structure has been proposed for the purpose of swiftly radiating heat from the battery cells at the central portion from which much heat is generated.
However, when the plurality of heat transfer sheets each having the same thickness are stacked, heat transfer through face-to-face is not swiftly performed between the sheets due to a contact thermal resistance between the sheets.
Therefore, a heat transfer performance of the heat transfer sheet in the inner side is relatively lower than a heat transfer performance of the heat transfer sheet in the outer side. Therefore, to swiftly radiate heat from the central portion, an unexpectedly large number of sheets need to be stacked. Since a stacking structure of the heat transfer sheets deteriorates energy density of the battery pack, it is difficult to apply the stacking structure of the heat transfer sheets to electric automobiles or hybrid automobiles.